A towing capacity chart can usually be found on the website of the trailer manufacturer. For example, if you are renting a U-Haul trailer, you may find a towing capacity chart on the U-Haul costumer service website. Or, if you own a Ford trailer, you can probably determine the trailer's capacity by using the most recent Ford towing capacity chart on the Ford website.
Most trailer manufacturers provide a new trailer towing capacity chart every year. If you consult a towing capacity chart on a company's website, make sure it is the most up-to-date chart the company has to offer. Otherwise, you may be using outdated information and damage your trailer and/or vehicle.
If you cannot find a towing capacity chart suited to your particular trailer, the next best thing is to find out the towing capacity of your vehicle. The most likely place to find the exact towing capacity of your vehicle is in your vehicle owner's manual. If your vehicle is not capable of towing a trailer, it is stated directly in your owner's manual. Most car companies also provide towing guides on their websites, so you won't be out of luck if you have lost your owner's manual or can't find the information you need.